Foams containing urethane groups obtained by reacting polyisocyanates with polyols are widely used in the field of insulation, for the manufacture of structural components and for upholstery purposes.
It is known that cold-hardening foams containing urethane groups can be produced in molds from compounds containing active hydrogen atoms, preferably polyols, and polyisocyanates, water and/or other blowing agents in the presence of emulsifiers, auxiliaries, catalysts and flameproofing additives. The object of including the emulsifiers and stabilizers in the reaction mixture is to homogenize the reaction components, to facilitate immediate foaming and to prevent the foams from collapsing on completion of gas formation. The catalysts are intended to ensure that the reactions which take place during foam formation are brought to the required equilibrium and take place at the correct rate.
Cold-hardening foams containing urethane groups which have the requisite physical foam properties are often produced from polyethers containing at least two hydroxyl groups, in which at least 10% of the OH-groups present are primary OH-groups. Such polyethers preferably have molecular weights of from about 750 to 100,000, preferably from 4000 to 10,000, and can be used in combination with "special" polyisocyanates.
Examples of these special polyisocyanates are the so-called "modified polyisocyanates", such as solutions of polyisocyanates containing biuret groups in polyisocyanates free from biuret groups or solutions of polyisocyanates containing at least two NCO groups and at least one N,N'-disubstituted allophanic acid ester group in polyisocyanates free from allophanic acid ester groups. Other kinds of modified polyisocyanates include solutions of reaction products of diisocyanates and divalent or polyvalent compounds containing hydroxyl groups in polyisocyanates free from urethane groups; and solutions of polyisocyanates containing more than one NCO-group and at least one isocyanuric acid ring in polyisocyanates free from isocyanurate groups or any mixture of these solutions.
The above mentioned cold-hardening foams have a disadvantage in that they show faults in the form of bubbles below the surface of the foam which can also spread into the interior of the molding. This is particularly true during in-mold foaming. This is a particularly unfavorable phenomenon both for the furniture industry and for the motor vehicle industry because the bubbles thus formed are distinctly visible in fine covering materials. It is not possible to eliminate these faults by using standard commercial-grade polysiloxane/polyalkylene oxide copolymers, because irreversible shrinkage occurs, even in the presence of very small quantities of stabilizers. Thus, foams result which are totally unsuitable for practical application.
Until now, a relatively large number of additives such as emulsifiers, stabilizers and various activators for the blowing and crosslinking reaction, have had to be used in the production of these cold-hardening molded foams. Such methods are necessary to produce the required properties such as an open-pore structure, favorable skin texture, uniform cell structure, high tensile strength, elasticity and loadbearing strength without permanent deformation. This can lead to a number of practical difficulties, such as the chemical incompatibility of the additives with each other, or the incompatibility of the additives with the polyols and/or isocyanates used for foaming. One large difficulty is the frequent insolubility of the additives in the polyols which results in inadequate stability during storage as a result of so-called exudation. Another disadvantage common to the numerous additives is their limited effectiveness in a certain recipe. In the event of a change in recipe, different additives or a greater number of additives have to be used.